Deployment of passive distribution systems, such as optical fiber, in the local telecommunications loop has opened opportunities for new types of services. Most of the new services target data transmission instead of voice. The major difference in the network requirements for digital data transmission versus voice transmission is in the quality of the transmission. The demand for high quality transmission and low outage time is forcing the new generation of systems to protect equipment as well as the passive distribution facilities.
The term "performance monitoring" is related to the quality of transmission over a network. One measure of a network's performance or transmission quality is the bit error rate (BER). Because of imperfections in the network and environmental conditions some data errors inevitably occur. However, equipment failures such as breakages, power loss, etc., can also cause data transmission errors to be introduced. BER is a measure of the number of errors which occur in a certain number of bits of transmission. For example, in a typical network the maximum acceptable BER is 10.sup.-10. This value means that one transmission error is allowed every 10.sup.10 bits. If the BER is greater than 10.sup.-10 the quality of transmission is not acceptable.
In order to determine the BER means must be provided to detect errors that occur. Another desirable feature is a mechanism to pinpoint the locations of the equipment failure which caused the errors so that equipment and protection facility switching can be achieved, or appropriate maintenance procedures performed.
A point-to-multipoint passive distribution network configuration consists of a head end connected to a single data path which splits into multiple branches, each branch associated with a unique remote unit. Communication between the head end and the multiple remote units is multiplexed on a passive distribution network (PDN) and each remote unit is programmed to extract and send data in a unique time slot. This means that all remote units share the single, or "feeder" section of the network, and that each have a dedicated branch, or "distribution" section of the network associated with it. One exemplary passive optical system is described in U.S. Pat. No. 4,977,593, to Ballance, issued Dec. 11, 1990 and assigned to British Telecommunications, which is incorporated herein by reference.
Performance monitoring of such a configuration represents a major challenge because the indication of an error in the error code does not itself identify the particular equipment which is causing the errors. Because the multiple remote units share the feeder section of the PDN, in existing systems it is not possible to identify whether an error was introduced in the shared feeder, in a dedicated distribution branch or in one of the remote units themselves.
Typically, the location of an equipment failure is determined by downing all or part of the system and performing interactive diagnostic tests between the head end and the multiple remote units. This method, however, results in an undesirable and severe degradation in performance of the system as a whole. There is therefore a need in the art for a means of monitoring the performance of a PDN connected in a point-to-multipoint configuration, which can isolate the location of equipment failures with minimal impact on overall system performance.